1. Technical Field of the Invention
The present invention relates in general to the mobile radio communications field and, in particular, to an improved method and system for determining the position of a mobile radio terminal.
2. Description of Related Art
In the cellular communications field, it has become increasingly important to be able to determine the position of mobile radio terminals. As such, authorities responsible for defining mobile radio communications system specifications and standards are currently involved in the process of specifying the accuracies required in performing mobile terminal position determinations. The most successful methods used so far to determine the position of mobile terminals are based on measurements of signal propagation times, which in turn, are then used to derive distances. These propagation time measurements are made on either the uplink (base station measurements of transmissions from a mobile terminal) or downlink (mobile terminal measurements of transmissions from a base station).
For example, World Intellectual Property Organization Patent Application Publication No. WO 96/35958 to Ghosh et al. ("Ghosh") discloses a method and system for determining the position of a mobile terminal in a Code Division Multiple Access (CDMA) communications system (e.g., in accordance with the IS-95 standard). The Ghosh application teaches a method by which measurements of the absolute time of arrival (TOA) of a signal transmitted by a mobile terminal are made in at least two base stations. These TOA measurements are converted to distances. Triangulation is used to determine the mobile terminal's position. However, a problem with the disclosed TOA method of position determination is that it requires the use of a highly accurate or "exact" time reference (e.g., as provided by the space-based Global Positioning System or GPS).
Patent Cooperation Treaty (PCT) Patent Application Serial No. PCT/SE97/00219 (and related U.S. patent application Ser. No. 08/799,039) to Lundqvist et al. ("Lundqvist") discloses a method and apparatus for determining the position of a mobile terminal in an unsynchronized environment (e.g., without using an "exact" time reference). Instead, a plurality of fixed location "reference" radio terminals whose positions are known are used to make downlink propagation time measurements. The relative transmission time offset between base stations is determined and used to derive the position of the mobile terminal.
PCT Patent Application Serial No. PCT/SE96/03561-3 (and related U.S. patent application Ser. No. 60/028,345) to B. Bergkvist et al. ("Bergkvist") discloses a method and apparatus for determining the position of a mobile terminal in a cellular mobile radio system such as, for example, the Global System for Mobile Communications (GSM). A mobile terminal is ordered to perform a sequence of handovers to several target base stations. As such, the mobile terminal transmits an access burst to a target base station. However, that target base station does not transmit a confirmation message that the access burst was received. The mobile terminal then reverts back to its serving base station. That target base station uses the received access burst to measure the round-trip propagation delay (base station-mobile terminal-base station). Consequently, a time reference signal is not needed to derive the mobile terminal's position.
A disadvantage of the method disclosed in the above-described Ghosh application is that the base stations are required to use a global time reference, such as a GPS signal, to accurately determine a mobile terminal's position. Similarly, although the method disclosed in the Lundqvist application avoids the use of a global time reference, instead it uses a complex system of fixed location "reference" radio terminals whose positions are known, in order to derive relative base station timing offsets. A disadvantage of the Bergkvist application is that it uses measurements of round-trip propagation delays from performing a sequence of aborted handovers. This method takes a considerable amount of time to complete, and it creates substantial disturbances by transmitting individual access bursts to several base stations. However, these access bursts are generated only for the purpose of determining the position of the mobile terminal.
Notably, it is desirable to determine the position of a mobile terminal without the use of complex time references, "reference mobile terminals, and "disturbing" aborted handovers, and instead be able to utilize the basic functions of the cellular mobile radio system. As described below, the present invention successfully provides this capability and resolves the above-described problems.